High-concentration Nitrogen addition enhances tree-level stomatal conductance and water consumption of Cunninghamia lanceolata in southern China

Reactive Nitrogen (N) deposition has increased in southern China, potentially influencing forest carbon and water exchange processes. Cunninghamia lanceolata (Lamb.) Hook dominates as the most extensively planted and economically fast-growing timber species in subtropical China, but how C. lanceolata responds to increased N deposition remains incompletely understood. In this study, we analyzed the responses of water utilization to increased N deposition at a manipulative experiment with N addition in a C. lanceolata plantation. Four treatments were established including N addition of 25 kg ha-1 year-1 (N1; low concentration), 50 kg ha-1 year-1 (N2; medium concentration) and 100 kg ha-1 year-1 (N3; high concentration) applied as NH4Cl solution, and control treatment (CK). Results showed that N3 treatment significantly increased leaf N content. N3 treatment enhanced intrinsic water-use efficiency as evidenced by leaf carbon isotope composition (δ13C), and leaf-level stomatal conductance as indicated by leaf oxygen isotopic composition (δ18O). Similarly, N3 treatment enhanced sap flux density (Js) and canopy stomatal conductance (Gc) of C. lanceolata during growing season. However, N1 and N2 treatments exerted minimal impacts on Js and Gc, suggesting there existed a dose-response relationship between external N input amount and Js (or Gc). Moreover, high-level N addition enhanced the sensitivity of Js and Gc to vapor pressure deficit and photosynthetically active radiation. Overall, C. lanceolata are more likely to enhance resource acquisition ability, and exhibit higher water consumption under future scenarios of increased N deposition.